Driver assistance systems make driving a vehicle easier in everyday situations, such as during parking, driving in columnar traffic, but also in exceptional situations such as avoiding an accident. To do this, the driver assistance systems require reliable radar systems which determine the distance from an obstacle and perhaps its speed relative to the vehicle.
FMCW radars (frequency-modulated continuous wave radar) are expediently used for this. The FMCW radar emits an output signal having other frequencies changing monotonically according to a plurality of adjustable modulation rates. The components of the emitted signal reflected by a potential obstacle are received by the radar. The instantaneously received and the instantaneously emitted signals differ in their frequency as a function of the propagation time of an emitted signal to the potential obstacle and the instantaneous modulation rate of the frequency of the emitted signal. From the difference of the frequencies, an evaluation unit determines the propagation time, and consequently, the distance from the potential obstacle.
A frequency variation also comes about, conforming to the Doppler effect, in response to the relative motion of the obstacle with respect to the radar. An evaluation of this frequency shift enables determining the relative speed.
Since a frequency shift may be attributed both to the separation distance and the relative speed, a single measurement for these two variables is not determinate.
Thus a second measurement is carried out. The modulation rate in the second measurement is different from the modulation rate in the first measurement. Because of the independence of the variation in the modulation rate of the frequency of the emitted signal, an identification of the contribution of the relative speed to the frequency shift is identifiable for the distance, in contrast to the signal propagation time measurements.
The aforementioned method requires that the first measurement be uniquely assignable to the second measurement. As long as there is only one obstacle, this is trivially possible. However, considerable difficulties arise if signals are received from a plurality of objects at different distances and/or having different relative speeds. An assignment of successive measurements to an object requires costly search algorithms, which, among other things, follow the trajectory of an object.